The present invention relates to systems for farming fish in cages and, more particularly, to fish farming systems having a cage which can be submerged and refloated as desired.
Considerable efforts have been made in an attempt to supply the rapidly increasing worldwide demand for fish protein. In addition to increasingly sophisticated open sea fishing, a significant fraction of the fish on the market today are raised and harvested using modem aquaculture techniques. Various fish farms have been successfully operating in large man-made pools. However, these farms are expensive to build and operate and do not always make it possible to reproduce optimal conditions for the growth of the fish.
More recently, fish farming has been increasingly carried out in large cages which are made to float near or at the water surface just offshore (hereinafter "nearshore") in seas, lakes or other natural bodies of water. A fish cage system includes one or more large cages which are typically constructed of a rigid frame of some suitable shape and covered by netting which allows water to flow freely into and out of the cage but which is of sufficiently fine mesh as to retain the fish inside the cage.
The advantage of such fish cage systems is that they do not take up scarce real estate and do not require the building of an expensive pool. Furthermore, the water conditions (e.g., salinity, temperature, oxygen content, and the like) approximate natural conditions in the open body of water and may be more optimal for the growth of the fish than conditions simulated in man-made onshore pools.
While the nearshore deployment of such fish cages is convenient in terms of accessibility, such deployment suffers from certain disadvantages. As nearshore aquaculture develops there is an increasing shortage of quality sites in which to locate additional cages. Many sites suffer from oxygen depletion caused by fish waste and uneaten fish food as well as from industrial, agricultural and domestic runoffs from the nearby shore.
It is therefore often advantageous to avoid onshore locations and to locate the cages farther offshore, in areas which are not adversely affected by runoffs and where the greater water circulation serves to dilute fish farm wastes.
However, locating fish cage systems in locations which are remote from the shore poses certain problems. Chief among these is the need to ensure the seaworthiness of the fish cage system in conditions, such as large waves and strong winds during storms, which may be much more severe than those experienced by nearshore structures.
Furthermore, it is known that during storms when the water near the water surface is particularly turbulent, fish, which normally spend most of the their time near the water surface where the supply of oxygen is most abundant, tend to temporarily relocate themselves away from the surface to depths where the water is relatively unaffected by the storm and thus avoid damage and stress to themselves.
To minimize or eliminate damage to both the fish and the cages, several fish cage systems have been developed which make it possible to submerge the fish cage to a certain depth when desired, e.g., prior to the onset of a storm, to avoid cold surface water and/or surface ice in winter and hot surface water in summer, or to avoid various toxic contaminants, such as toxic plankton blooms or an oil spill.
One such system is the Trident (Trademark) Sea Cage System marketed by Innovation & Development Partners Inc. of Nepean, Ontario, Canada. The Trident system, shown schematically in FIGS. 1a and 1b, includes a spherical cage 10 and a mooring system. The mooring system includes a large mooring anchor 11 through which the mooring cable 12 is led (apparently through a suitable pulley, or similar mechanism, which is not shown in the figures) to a modified crown buoy 14 and thence to the second mooring anchor 16. Tension on mooring cable 12 is maintained by the buoyancy of crown buoy 14 which includes two chambers. Under normal conditions, the lower chamber of crown buoy 14 provides permanent flotation designed to tension mooring cable 12 so as to submerge the floating cage 10 to the desired degree (FIG. 1a). In this condition, the upper chamber of crown buoy 14 remains completely filled with water. When it is desired to fully submerge cage 10, water is blown out of the upper chamber of crown buoy 14 using compressed air which greatly increases the buoyancy of crown buoy 14 causing it to rise and to submerge cage 10 to a predetermined depth (FIG. 1b).
Another system is the Atoll 250 marketed by Aquavar of France. A portion of the Atoll 250 system is depicted schematically in FIG. 2. Here, as in the Trident system described above, a number of large mooring anchors 20, 22 and 24, are used and are provided with pulleys 26, 28 and 30 through which one or more mooring cables 34 can move. A buoy 32 is provided to alternately take in and release mooring cable 34 which, in turn, determines the position of fish cage 36.
A disadvantage of both systems is that they use large permanent mooring anchors which do not allow the system to be easily redeployed in other locations as the need arises. Furthermore, each of the systems requires pulleys or similar mechanisms located near the permanent mooring anchors near the sea floor. Such devices typically require expensive maintenance, involving the services of skilled divers. The need to access the mechanisms near the sea floor puts a practical limit on the maximum depth at which such systems can be usefully deployed.
Typically, such systems are not deployed in waters which are deeper than about 30 meters. While such depths may be sufficient in some circumstances, in other cases it may be desirable to lower a fish cage to a greater depth. Furthermore, it may, in some cases, be desirable to deploy fish cages in waters which are considerably deeper than 30 meters, without regard to the depth to which it is desired to submerge the cage.
There is thus a widely recognized need for, and it would be highly advantageous to have, a fish cage system which would be easily deployed in waters of virtually any depth, which would allow the fish cages to be submerged in a controlled manner to virtually any depth and which would be easily and readily redeployed to other locations.